There is a need for a method and system capable of efficiently and effectively filtering pollutants from exhaust gases. Although there are a number of devices available which are useful for filtering exhaust gases from diesel engines, each of these devices is incapable of providing an effective method for reducing pollutants cost effectively for the reasons described herein.
In a diesel engine, air is drawn into the cylinders and is compressed by the pistons at compression ratios as high as 25:1, much higher than used for spark-ignited combustion engines. Near the end of the compression stroke, diesel fuel is injected into the combustion chamber through an injector (or atomizer). The fuel ignites from contact with the air that due to compression has been heated to a temperature of about 700-900° C. The resulting combustion causes increased heat and expansion in the cylinder which increases pressure and moves the piston downward. A connecting rod transmits this motion to a crankshaft to convert linear motion to rotary motion for use as power in a variety of applications. Intake air to the engine is usually controlled by mechanical valves in the cylinder head. For increased power output, most modern diesel engines are equipped with a turbocharger, and in some derivatives, a supercharger to increase intake air volume. Use of an aftercooler to cool intake air that has been compressed, and thus heated, by the turbocharger increases the density of the air and typically leads to power and efficiency improvements.
In general, diesel emissions are bi-products of diesel combustion. This can be a function of injection within the engine. For example, advancing the start of injection (injecting before the piston reaches top of dead center) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in higher emissions of oxides of nitrogen oxides through higher combustion temperatures. At the other extreme, delayed start of injection causes incomplete combustion and emits visible black smoke made of particulate matter and unburned hydrocarbon. While many diesel emissions are problematic, the most highly regulated diesel emissions are:    1. Diesel Particuiate Material (“PM”, or “DPM”) (also referred to as “Diesel Particulate Matter”, “Particulate Material”, or “Particulate Matter”): Particulate matter is an aerosol comprised of complex physical and chemical structures. Particulate matter contributes to the greenhouse effect, it causes grave environmental damage, and it seriously affects human health. Particulate matter is primarily responsible for the black smoke normally associated with diesel exhaust. It is also a primary source of urban smog.    2. Nitrogen Oxides (NOx): Nitrogen Oxides are highly active ozone precursors and account for a large component of visible smog. Besides particulate matter, nitrogen oxides are one of the most pollutive diesel emissions.    3. Hydrocarbons (HC): The production of hydrocarbons is often a result of the inefficient combustion of fuel and engine lube oils. In the atmosphere, hydrocarbons undergo photochemical reactions with nitrogen oxides leading to formation of smog and ground level ozone.    4. Carbon Monoxide (CO): This is a highly toxic greenhouse gas that is poisonous to humans and is a contributor to global warming.
Examples of non-regulated bi-products of diesel combustion include polynuclear aromatic hydrocarbons, aldehydes, sulfur dioxide, nitrous oxide, and metal oxide.
Inefficient combustion of diesel fuel produces emissions that pollute the environment and harm human health. The environmental consequences of particulate material emissions include air pollution, water pollution, acid rain, acidification of waterways, deforestation, smog, reduced atmospheric visibility, crop degradation, global warming, and climate forcing. In addition, the human health consequences of particulate material emissions include cardiovascular disease, respiratory disease, cancer, fibrosis, allergic responses, reduced pulmonary function, worsening of asthmatic symptoms and occurrences, increased morbidity, and premature death. Moreover, a number of internationally publicized studies demonstrate a high correlation between ambient particulate material and increases in adverse health outcomes such as respiratory hospital admissions, emergency room visits, restricted activity days, respiratory symptoms for adults, lower respiratory tract illnesses for children, asthmatic attacks, chronic diseases, and mortality.
Although conventional diesel emission filtration technologies are numerous, there are essentially two categories into which all such technologies fall:    1. Catalyzed Diesel Particulate Filters (“CDPFs”): catalyzed diesel particulate filters are referred to by many different names. Some of the most commonly used—and misused—are: “catalytic converters,” “catalytic Reactors,” “catalytic purifiers,” “exhaust purifiers,” “trap filters,” “diesel traps,” “exhaust scrubbers,” “catalyst filters,” “catalyzed wall-flow filters,” “wall-flow filters,” and “catalytic mufflers.”    2. Diesel Oxidation Catalysts (“DOCs”): diesel oxidation catalysts are also commonly referred to as “oxidation catalysts,” “flow-through catalysts,” and “flow-through devices.”
Both catalyzed diesel particulate filters and diesel oxidation catalysts employ the same basic method to achieve the reduction of particulate materials; they utilize heat to “oxidize” or bun the particulate material. In most cases, the heat from the engine's exhaust system is used to achieve oxidation. The reoccurring process of oxidation is also often referred to “regeneration” because the process of oxidation not only reduces particulate material emissions, it also regenerates the catalytic device's filtration capacity.
In order for the process of regenerative oxidation to occur, high temperatures, normally between 250° and 350° C., must be attained and preferably sustained during operation. In many operating conditions, attaining sufficiently high temperatures can prove difficult or unattainable. Catalytic devices (CDPF's and DOC's) employ precious metals such as platinum, palladium and rhodium as catalysts to lower the minimum temperatures necessary to achieve “light off”, the point at which oxidation of the particulate material is initiated. Manufactures use these highly conductive, and very expensive, metals to coat or impregnate the substrate surfaces of their catalytic devices.
The catalytic devices discussed above can generally be described as either active or passive. Catalytic technologies which rely on heat from an engine's exhaust system in order to achieve oxidation are frequently referred to as “passive” catalytic devises. Other systems may incorporate fuel burners, electric heating elements, and fuel-borne additives which aid in attaining the temperatures at which oxidation occurs. Technologies which employ these types of components are often referred to as “active” catalytic devices.
For purposes of eliminating potential confusion, it should be noted that some manufacturers define catalyzed diesel particulate filters which only contain precious metal catalysts as “active” devices, even though these devices rely solely upon the heat contained in an engine's exhaust to achieve oxidation. This classification usually occurs when the manufacturer also produces a diesel particulate filter which contains no catalyst, i.e. a device which is in all other ways similar to a catalyzed diesel particulate filter, however; the device relies solely upon the heating of its component base metal to achieve temperatures sufficient to initiate oxidation. Because exhaust temperatures are commonly required to exceed 500° C. for these non-catalyzed devices to affect oxidation, their widespread use is significantly restricted.
The primary difference between catalyzed diesel particulate filters and diesel oxidation catalyst technologies is that catalyzed diesel particulate filter technologies physically trap and store particulate material—usually by using catalyzed ceramic, cordierite or silicon carbide wall flow monoliths, or ceramic fiber or ceramic cartridge filters. Once the particulate material becomes trapped, it is oxidized and particulate material emissions are reduced.
Conversely, diesel oxidation catalyst technologies do not trap particulate material emissions. Rather, particulate materials “pass-through” the internal structures of these devices. When exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles are oxidized, thereby reducing total particulate material emissions.
There are a number of other differences between catalyzed diesel particulate filters and diesel oxidation catalyst technologies as well. For example, catalyzed diesel particulate filters can achieve particulate material filtration rates of ≧90% given specific, controlled operating conditions. Moreover, catalyzed diesel particulate filters reduce each sub-category of particulate material (i.e. solid inorganic fractions, solid organic fraction and sulfate particulates). It is necessary to note however, the application and effectiveness of catalyzed diesel particulate filters technology is significantly constrained by the following limitations:                Catalyzed diesel particulate filters are very expensive. The California Air Resources Board provides cost-range information for DPF's corresponding to the following engine capacitates:                    100 horsepower: US$5,000-US$7,000            275 horsepower: US$6,900-US$9,000            400 horsepower: US$10,000 average            1,400 horsepower: US$32,000+                        Catalyzed diesel particulate filters are incapable of affecting particulate material emissions reductions when using fuels that exceed 150 ppm Sulfur.        Catalyzed diesel particulate filters performance is adversely affected by insufficient operating temperatures.        In less-than-optimal conditions, catalyzed diesel particulate filters are prone to clogging and failure. When failure occurs, the potential for engine damage or destruction is significant.        Because catalyzed diesel particulate filters can create significant engine back pressure, expensive engine recalibrations are often required upon their installation.        catalyzed diesel particulate filters often need to be equipped with expensive electronic back pressure monitoring devices, such as data loggers.        Because passive catalyzed diesel particulate filters regeneration is entirely dependent on operating temperature, passive catalyzed diesel particulate filters do not work under “low load” conditions.        “Active” components in catalyzed diesel particulate filter technologies significantly increase catalyzed diesel particulate filters unit price and complexity.        Catalyzed diesel particulate filters do not work well on older engines.        Catalyzed diesel particulate filters can become a source of hazardous zinc, sulfuric, calcium, and phosphorus ash particulate.        Catalyzed diesel particulate filters can reduce engine performance.        Catalyzed diesel particulate filters often produce fuel economy penalties.        
According to the United States Department of Energy (USDOE), fuel sulfur has significant effects on post-filter total particulate material emissions, and, as fuel sulfur levels increase, catalyzed diesel particulate filter reduction efficiencies decreases to a point where they actually becomes a source of particulate emissions when using fuels with sulfur concentrations ≧150 PPM.
Tests conducted by the USDOE report that catalyzed diesel particulate filters that achieved 95% reductions of particulate material emissions when using fuels with 3 ppm sulfur concentrations had their filtration efficiencies reduced to only 74% when using fuels with 30 ppm sulfur concentrations. Further, these same devices were reduced to particulate material filtration rates of 0% to −3% when using fuels with 150 ppm sulfur concentrations, and they experienced total particulate material emissions increases of 122% to 155% when using fuels with sulfur concentrations ≧350 ppm.
Moreover, the Natural Resources Defense Council (NRDC) has stated that catalytic technologies can not work properly if there is sulfur in the fuel—and in some cases, sulfur in the fuel will render the catalytic filtration equipment and even the vehicle inoperable.
By comparison, diesel oxidation catalyst technologies are generally less expensive than catalyzed diesel particulate filter technologies, and because diesel oxidation catalysts are “flow through”, instead of “wall flow” devises, they do not have the same propensity to create engine back pressure, clog and/or cause potential engine damage like their catalyzed diesel particulate filter counterparts. Diesel oxidation catalysts can achieve particulate material filtration rates between 19% and 50%. However, the application of diesel oxidation catalyst technology is constrained by the following:                Diesel oxidation catalysts are too expensive for wide-spread application. The California Air Resources Board provides cost average information for diesel oxidation catalysts corresponding to the following engine capacitates:                    275 horsepower: US$2,100            400 horsepower: US$20,000+            The Everett School District in Washington state reported an average per-unit-cost of US$2,500 per DOC for each bus in its fleet                        Diesel oxidation catalyst reduction of total particulate material is significantly reduced when using fuels with high sulfur fuels.        Diesel oxidation catalysts do not filter solid organic fraction sometimes called “dry”) particulate and dry particulates typically comprise the majority of total particulate material.        Diesel oxidation catalysts do not work well on older engines.        Diesel oxidation catalyst effectiveness is extremely dependent upon operating temperatures.        When operating at higher temperatures, diesel oxidation catalysts oxidize sulfur oxides, and in doing so become generators of sulfuric acid. When this occurs, diesel oxidation catalysts create a net increase total particulate material emissions by increasing production of sulfate particulates at rates that offset soluble organic fraction reductions        
The University of Washington's Extension Energy Program has stated that diesel oxidation catalysts can oxidize sulfur dioxide to form sulfate particulates (sulfuric acid (H2SO4)). Therefore, high sulfur content fuels can increase total particulate emissions via the production of sulfuric acid, which can offset soluble organic fraction (sometimes called “wet” particulate material) reductions.”
The United States Department of Energy has found statistically significant increases in particulate material with high sulfur fuel due almost exclusively to the increase in the SO4 fraction of the total particulate material. At this high exhaust temperature (405° C. at catalyst inlet), the diesel oxidation catalyst accelerates the conversion of SO2 to SO3, thereby increasing the SO4 fraction of the particulate material. As expected, the effect is seen only with the higher sulfur (150 ppm and 350 ppm sulfur content) fuels. With the 350 ppm sulfur content fuel, post catalyst particulate material emissions were approximately 200% higher than those measured without an active catalyst.
Despite the promoted efficiency of the methods and systems of the prior art, many are impracticable from the commercial point of view for the reasons set forth above. Moreover, the use of fuel with low concentration of sulphur (below 130 ppm) is an essential factor in the employment of catalytic regeneration filters. In Brazil and in the majority of the countries, the diesel is sold with 2000 ppm of sulphur. Therefore using the catalytic regeneration filters in diesel that contains more than 300 ppm of sulphur, turn the filters into a source of pollution.